The following patents and patent applications are believed to represent the current state of the art relevant to the present invention:
U.S. Pat. Nos. 4,044,227; 4,268,179; 4,794,238; 4,844,509; 5,468,949; 5,600,121, 5,621,864, 5,825,012; 5,828,048; 5,828,049; 5,988,508; 5,996,895; 6,070,801, 6,127,928, 6,206,286; 6,252,508; 6259,408; 6,265,977; 6,283,375; 6,285,342, 6,286,763, 6,317,044.